Chemical ionization mass spectrometry of carbamates. 1—N-phenylcarbamates—propham and chlorpropham

Abstract

Under ammonia and methane chemical ionization conditions, using a liquid chromatographic moving belt interface for sample introduction, molecular weight information has been observed for two representative N-phenykarbamates, propham (isopropyl N-phenylcarbamate) and chlorpropham [isopropyl N-(3 -chlorophenyl)carbamate]. Displacement reactions can occur in which the side-chain is expelled as a neutral olefin via a hydrogen rearrangement. The temperature dependence of these spectra has been studied and the relative concentration of [MH 1' ions increases with decreasing temperature. INTRODUCTIONAnalysis of carbamate pesticides by gas chromatography has been seriously hampered by their reported thermal lability.17' Attempts at analytical separation and identification of N-phenylcarbamates have employed derivatization3 or liquid chromatogra hy4 to avoid thermal decomposition to isocyanates.' More recently, liquid chromatography/mass s ectrometry (LC/MS) has been successfully emp1oyed7,'in the quantitative analyses of carbamates and ureas. While the use of the moving belt interface employed in these previous studies has avoided thermal decomposition via flash vaporization from a polyimide belt, a full explanation of the resultant fragmentation processes was not forthcoming. As a first preliminary step to developing an analytical protocol for residue work, the characterization of such compounds is commonplace in ascertaining the appropriate temperature, reagent gas and ions to be monitored. In the course of characterizing two representative carbamates, propham (1) and chloropropham (2) by LC/MS, certain spectral elements emerged which required detailed study and comment. To assist in this structural investigation, the corresponding N-methylated chlorpropham (3) was prepared and also studied. In this paper, we now report an SE2 reaction as responsible for the major fragment ion in these two compounds via a hydrogen rearrangement mechanism. t Author to whom correspondence should be addressed. EXPERIMENTALAll spectra were obtained on a Finnigan Model 3300 quadrupole mass spectrometer equipped with a chemical ionization (CI) source, INCOS data system and Finnigan liquid chromatographic moving belt interface. Kapton belts were used with a variable flash vaporizer temperature, a CI source temperature of 180°C and a source pressure of 0.8 Torr (adjusted to maximize the intensity of the CH: or the NH: signals). Compounds were spotted on the moving belt using 2 ~1 of standard solutions made up at the concentration level of 1 mg m1-I.The N-methyl derivative of chlorpropham was prepared by alkylation with iodomethane and sodium hydride. The isolated product was estimated by mass spectrometry to be greater than 95% pure and was used without further purification. Figure 1 illustrates the mass spectra obtained for propham (1) using CH4, C2H4 and NH3 as reagent gases. At 220 "C a protonated molecular ion at rn/z 180 is observed with a low relative abundance. The major fragment ion at m / z 138 represents a loss of 42 u from [MH]+. Whi...

Ion formation of N-Methyl carbamate pesticides in thermospray mass spectrometry: The effects of additives to the liquid chromatographic eluent and of the vaporizer temperature

“…On the basis of our data, and assuming equilibrium gas-phase chemistry, we conclude that the PA of most carbamates must be close to that of ammonia (854 kJ/mol [50]) and even closer to that of nicotinic acid (not available). The fact that complex formation occurs with most carbamates is in line with the idea [13,17,251 that the carbamate group provides the site of protonation. Moreover, the carbamates that do not generate complex ions (aminocarb, carbendazim, pirimicarb) have structural features that may provide a different site of protonation: a dimethyl amino group in aminocarb and an aromatic ring nitrogen in carbendazim and pirimicarb would make these compounds more basic.…”

Chemical ionization mass spectrometry of carbamate pesticides: a major dissociation pathway

“…Due to low barriers, this cation can now rotate to prepare to receive the amide hydrogen for subsequent transfer. Such an electrostatic situation had already been strongly suggested to explain the nonexclusivity of hydrogen transfer observed in both propham and chlorpropham (1) where the loss of a neutral olefin was predominant. More recently, however, Masquestiau et al (9) have also invoked the same qualitative picture of ion/molecule complexes to explain the fragmentation of a series of phenyl ethers where the resultant end products are either protonated dienes or protonated phenol.…”

“…Deuterated methane (99 atom %) was purchased from Merck and Co. Deuterated Acetone (99.5 atom %) was obtained from Aldrich Chemical Co. Carbamate pesticides were obtained as analytical standards from the EPA repository and used without further purification. The N-methyl analogues of promecarb and carbaryl were prepared as previously described (1).…”

“…At the other extreme, the em values for those binary solvents in which the strong HBD is added to a highly polar HBA base conform to simple linear functions in , and the methanol-acetonitrile and ethanol-acetonitrile pairs illustrate that condition (Table II) where the monomer of the alcohol forms a single and very stable hydrogen-bonded complex with the nitrile. For the latter cases, the structure breaking of the alcohol by the added highly polar HBA decreases the value of x in eq 1 (14).…”

“…The application of pattern recognition to mass spectra has been reviewed recently by Martinsen (1). Some of the previous chemometric investigations of mass spectra include factor analysis studies (2-7), primarily to determine the number of components in a mixture, cluster analysis and K nearest neighbor studies (8)(9)(10)(11)(12), discriminant analysis studies (13,14), and SIMCA principal component studies (12,15).…”